1. Technical Field
The invention relates in general to a rail-to-rail amplifier.
2. Background
Referring concurrently to FIG. 1A and FIG. 1B, FIG. 1A shows a circuit diagram of a traditional rail-to-rail amplifier, and FIG. 1B shows a schematic illustration showing operation voltage regions of a traditional rail-to-rail amplifier. In the rail-to-rail amplifier 10 shown in FIG. 1A, NMOS transistors N1 and N2 form an NMOS input pair, PMOS transistors P1 and P2 form a PMOS input pair, and the NMOS input pair and the PMOS input pair are respectively coupled to current sources 12 and 14.
When an input voltage Vi is less than threshold voltages of the NMOS transistors N1 and N2, the NMOS transistors N1 and N2 are turned off, so that a current of the current source 12 does not flow into an output stage circuit 16; meanwhile, the PMOS transistors P1 and P2 are turned on, so that a current of the current source 14 flows into the output stage circuit 16 for operations. In addition, when the input voltage Vi is larger than threshold voltages of the PMOS transistors P1 and P2, the PMOS transistors P1 and P2 are turned off, so that the current of the current source 14 does not flow into the output stage circuit 16; meanwhile, the NMOS transistors N1 and N2 are turned on, so that the current of the current source 12 flows into the output stage circuit 16 for operations. It can be obtained from FIG. 1B that the NMOS input pair and the PMOS input pair are turned on simultaneously at some input voltage regions of the rail-to-rail amplifier 10. Compared with a single input pair amplifier, the rail-to-rail amplifier 10 has an advantage of operating at full input voltage regions, but causes unnecessary power consumption.